ABSTRACT

Facilitation by nurse plants likely increases from outside the canopy to the center of the canopy as stresses decrease towards the center of the canopy. These stress gradients may be important in controlling plant distributions, with stress-tolerant species specializing outside the canopy or at the canopy edge, and stress intolerant species specializing at the center of the canopy. We tested if interactions with nurse plants control the distribution of understory species, and if plants species specializing in the understory environment experience higher physiological stress when grown outside the canopy than species specializing in the open environments. We tested these predictions in field sites in the arid environment of Saudi Arabia. We measured the environmental conditions, understory species abundance, and functional and physiological traits of species found under nurse plant Acacia gerrardii. We found that Acacia trees have an overall facilitative impact on the understory species. Species found more commonly under nurse tree canopies experience significant physiological stress when growing outside canopies. In contrast, species found more commonly outside canopies do not experience significant physiological stress when growing either under canopies or outside canopies. Our results demonstrate that differences in species ability to tolerate environmental stresses are important in structuring herbaceous plant communities under nurse plants in these extremely stressful environments.

摘要

护士植物对植物的促进作用可能会从树冠外部向树冠中心增加，因为应力会朝着树冠中心减小。这些胁迫梯度对于控制植物分布可能很重要，其中耐胁迫物种专门在冠层外部或在冠层边缘，而耐胁迫物种专门在冠层中心。我们测试了与护士植物的相互作用是否控制了林下物种的分布，以及专门用于林下环境的植物物种在冠层外生长时是否比专门用于开放环境的物种承受更高的生理压力。我们在沙特阿拉伯干旱环境中的现场测试了这些预测。我们测量了环境条件，林下物种的丰富度，金合欢。我们发现相思树对林下树种具有整体促进作用。当在室外树冠上生长时，在nurse树树冠下更常见的物种会遭受明显的生理压力。相反，在树冠下或树冠下生长时，在树冠外更常见的物种不会承受明显的生理压力。我们的结果表明，在这些极度胁迫的环境下，物种耐受环境胁迫的能力差异对于构造护士植物下的草本植物群落非常重要。